1. Field of the Invention
The present invention relates to a micro mirror unit and a method of making it. The micro mirror unit is an element incorporated e.g. in an optical switching device which switches optical paths between a plurality of optical fibers, or in an optical disc drive which records data onto an optical disc and/or reproduces data recorded on it.
2. Description of the Related Art
In recent years, optical communications technology is utilized widely in a variety of fields. In the optical communications, optical fibers serve as a medium through which optical signals are passed. When the optical signal passing through a given optical fiber is switched to another optical fiber, so-called optical switching devices are used in general. In order to achieve high quality optical communications, the optical switching device must have high capacity, high speed and high reliability in switching action. In view of these, micro mirror units manufactured by micro-machining technology is attracting attention as a switching element to be incorporated in the optical switching device. The micro mirror units enable the switching operation without converting optical signals into electric signals between the optical paths on the input side and the output side of the optical switching device. This is advantageous to achieving the desired characteristics mentioned above.
Micro mirror units are disclosed e.g. in Japanese Patent Laid-Open No. 4-343318 and No. 11-52278. Further, optical switching devices which use micro mirror units manufactured by micro-machining technologies are disclosed in the article xe2x80x9cMEMS Components for WDM Transmission Systemsxe2x80x9d (Optical Fiber Communication [OFC] 2002, pp.89-90 etc.
FIG. 21 outlines an ordinary optical switching device 500. The optical switching device 500 includes a pair of micro mirror arrays 501, 502, an input fiber array 503, an output fiber array 504, and a plurality of micro lenses 505, 506. The input fiber array 503 includes a predetermined number of input fibers 503a. The micro mirror array 501 is provided with the same plurality of micro mirror units 501a each corresponding to one of the input fibers 503a. Likewise, the output fiber array 504 includes a predetermined number of input fibers 504a. The micro mirror array 502 is provided with the same plurality of micro mirror units 502a each corresponding to one of the output fibers 504a. Each of the micro mirror units 501a, 502a has a mirror surface for reflection of light. The orientation of the mirror surface is controllable. Each of the micro lenses 505 faces an end of a corresponding input fiber 503a. Likewise, each of the micro lenses 506 faces an end of a corresponding output fiber 504a. 
In transmitting optical signals, lights L1 coming out of the output fibers 503a pass through the corresponding micro lenses 505 respectively, thereby becoming parallel to each other and proceeding to the micro mirror array 501. The lights L1 reflect on their corresponding micro mirror units 501a respectively, thereby deflected toward the micro mirror array 502. At this point, the mirror surfaces of the micro mirror units 501a are oriented, in advance, in predetermined directions so as to direct the lights L1 to enter their respective desired micro mirror units 502a. Then, the lights L1 are reflected on the micro mirror units 502a, and thereby deflected toward the output fiber array 504. At this point, the mirror surfaces of the micro mirror units 502a are oriented, in advance, in predetermined directions so as to direct the lights L1 into their respective desired output fibers 504a. 
As described, according to the optical switching device 500, the lights L1 coming out of the input fibers 503a reach the desired output fibers 504a due to the deflection by the micro mirror arrays 501, 502. In other words, a given input fiber 503a is connected with an output fiber 504a in a one-to-one relationship. With this arrangement, by appropriately changing deflection angles of the micro mirror units 501a, 502a, switching can be performed and the lights L1 can be deflected into different output fibers 504a. 
FIG. 22 outlines another ordinary optical switching device 600. The optical switching device 600 includes a micro mirror array 601, a fixed mirror 602, an input-output fiber array 603, and a plurality of micro lenses 604. The input-output fiber array 603 includes a predetermined number of input fibers 603a and a predetermined number of output fibers 603b. The micro mirror array 601 includes the same plurality of micro mirror units 601a each corresponding to one of the fibers 603a, 603b. Each of the micro mirror units 601a has a mirror surface for reflection of light and orientation of the mirror surfaces is controllable. Each of the micro lenses 604 faces an end of a corresponding one of the fibers 603a, 603b. 
In transmitting optical signals, light L2 coming out of the input fiber 603a passes through the corresponding micro lens 604 and is directed toward the micro mirror array 601. The light L2 is then reflected by a corresponding first micro mirror unit 601a, and thereby deflected toward the fixed mirror 602, reflected by the fixed mirror 602, and then enters a corresponding second micro mirror unit 601a. At this point, the mirror surface of the first micro mirror unit 601a is oriented, in advance, in a predetermined direction so as to direct the light L2 to enter a predetermined one of the micro mirror units 601a. Then, the light L2 is reflected on the second micro mirror unit 601a, and thereby deflected toward the input-output fiber array 603. At this point, the mirror surface of the second micro mirror unit 601a is oriented, in advance, in a predetermined direction so as to direct the light L2 to enter a predetermined one of the output fibers 603b. 
As described, according to the optical switching device 600, the light L2 coming out of the input fiber 603a reaches the desired output fiber 603b due to the deflection by the micro mirror array 601 and the fixed mirror 602. In other words, a given input fiber 603a is connected with an output fiber 603b in a one-to-one relationship. With this arrangement, by appropriately changing deflection angles of the first and the second micro mirror units 601a, switching can be performed and the light L2 can be deflected into different output fibers 603b. 
FIG. 23 is a perspective view, partly unillustrated, of a portion of a conventional micro mirror unit 700 for incorporation in such devices as the optical switching devices 500, 600. The micro mirror unit 700 includes a mirror-formed portion 710 having an upper surface provided with a mirror surface (not illustrated), an inner frame 720 and an outer frame 730 (partly unillustrated), each formed with come-like electrodes integrally therewith. Specifically, the mirror-formed portion 710 has ends facing away from each other, and a pair of comb-like electrodes 710a, 710b are formed respectively on these ends. In the inner frame 720 a pair of comb-like electrodes 720a, 720b extend inwardly, corresponding to the comb-like electrodes 710a, 710b. Also, a pair of comb-like electrodes 720c, 720d extend outwardly. In the outer frame 730 a pair of comb-like electrodes 730a, 730b extend inwardly, corresponding to the comb-like electrodes 720c, 720d. The mirror-formed portion 710 and the inner frame 720 are connected with each other by a pair of torsion bars 740. The inner frame 720 and the outer frame 730 are connected with each other by a pair of torsion bars 750. The pair of torsion bars 740 provides a pivotal axis for the mirror-formed portion 710 to pivot with respect to the inner frame 720. The pair of torsion bars 750 provides a pivotal axis for the inner frame 720, as well as for the associating mirror-formed portion 710, to pivot with respect to the outer frame 730.
With the above arrangement, in the micro mirror unit 700, a pair of comb-like electrodes, such as the comb-like electrode 710a and the comb-like electrode 720a, are opposed closely to each other for generation of static electric force, and take positions as shown in FIG. 24A, i.e. one of the electrode assuming a lower position and the other assuming an upper position, when there is no voltage applied. When an electric voltage is applied, as shown in FIG. 24B, the comb-like electrode 710a is drawn toward the comb-like electrode 720a, thereby pivoting the mirror-formed portion 710. More specifically, in FIG. 23, when the comb-like electrode 710a is given a positive charge whereas the comb-like electrode 720a is given a negative charge, the mirror-formed portion 710 is pivoted in a direction M1 while twisting the pair of torsion bars 740. On the other hand, when the comb-like electrode 720c is given a positive charge whereas the comb-like electrode 730a is given a negative charge, the inner frame 720 is pivoted in a direction M2 while twisting the pair of torsion bars 750.
As a conventional method, the micro mirror unit 700 can be made from an SOI (Silicon on Insulator) wafer which sandwiches an insulating layer between silicon layers. Specifically, first, as shown in FIG. 25A, a wafer 800 is prepared which has a layered structure including a first silicon layer 801, a second silicon layer 802, and an insulating layer 803 sandwiched between these silicon layers. Next, as shown in FIG. 25B, an anisotropic etching is performed to the first silicon layer 801 via a predetermined mask, to form the mirror formed portion 710, torsion bars 140, the comb-like electrode 710a and other members to be formed on the first silicon layer 801. Next, as shown in FIG. 25C, an anisotropic etching is performed to the second silicon layer 802 via a predetermined mask, to form the comb-like electrode 720a and other members to be formed on the second silicon layer 802. Note that for the sake of simplification of the drawings, each of the FIG. 25A through FIG. 25C gives only one sectional view, and each view includes a plurality of sections taken at different locations in the wafer 800.
However, according to the conventional method of manufacture as described above, the thickness of the wafer 800 is directly reflected on the thickness of the micro mirror unit 700. Specifically, the thickness of the micro mirror unit 700 is identical with the thickness of the wafer 800 which is used for the formation of the micro mirror unit. For this reason, according to the conventional method, the material wafer 800 must have the same thickness as the thickness of the micro mirror unit 700 to be manufactured. This means that if the micro mirror unit 700 is to be thin, the wafer 800 of the same thinness must be used. For example, take a case of manufacturing a micro mirror unit 700 having a mirror surface having a size of about 100 through 1000 xcexcm. In view of a mass of the entire moving part including the mirror-formed portion 710 and the inner frame 720, the amount of movement of the moving part, the size of the comb-like electrodes necessary for achieving the amount of movement, etc considered comprehensively, a desirable thickness of the moving part or the micro mirror unit 700 is determined. In this particular case the desirable thickness is 100 through 200 xcexcm. As a result, in order to manufacture the micro mirror unit 700 having such a thickness, a wafer 800 having the thickness of 100 through 200 xcexcm is used.
According to the conventional method, in order to manufacture a thin micro mirror unit 700, a correspondingly thin wafer 800 must be used. This means that the greater diameter the wafer 800 has, the more difficult to handle the wafer. For instance, take a case in which a micro mirror unit 700 is to be manufactured from an SOI wafer 800 having a thickness of 200 xcexcm and a diameter of 6 inches. Often, the wafer 800 is broken in a midway of the manufacturing process. After formation of the predetermined structural members on the first silicon layer 801 as shown in FIG. 25B, strength of the wafer 800 is decreased, making especially difficult to handle the wafer during the machining on the second silicon layer 802. Thinness of the wafer 800 limits, as has been described, the size of the flat surface of the wafer due to handling difficulties. Further, the limitation on the size of the flat surface of the wafer places a limitation on the manufacture of micro mirror array chips. Specifically, when the micro mirror array chips are manufactured by forming a plurality of micro mirror units in an array pattern on a single substrate, the size of the array is limited.
FIG. 26 shows a micro mirror unit 700 mounted on a wiring substrate. In the figure, the micro mirror unit 700 shows a section taken on lines XXVIxe2x80x94XXVI in FIG. 23. According to the conventional micro mirror unit 700 in FIG. 23, the moving part including the mirror-formed portion 710 and the inner frame 720 has the same thickness as the outer frame 730. For this reason, when the micro mirror unit 700 is mounted onto the wiring substrate 810, in order to allow the moving part to move properly, a spacer 811 must be provided as shown in FIG. 26 between the wiring substrate 810 and the outer frame 730. By providing the spacer 811 having a sufficient thickness between the micro mirror unit 700 and the wiring substrate 810, it becomes possible to avoid a situation that the moving part makes contact to the wiring substrate 810 to become unable to move. In view of a mounting process of the micro mirror unit 700 onto the wiring substrate 810, it is not efficient to provide the spacer 811 separately.
The present invention has been proposed under the circumstances described above. It is therefore an object of the present invention to provide a micro mirror unit capable of reducing the limitation on the size of the flat surface of the wafer used for the manufacture. Another object of the present invention is to provide a method of making such a micro mirror unit.
According to a first aspect of the present invention, there is provided a micro mirror unit comprising: a moving part including a mirror portion; a frame; and a torsion bar connecting the moving part to the frame. The moving part, the frame and the torsion bar are formed integral from a common material substrate. The frame includes a portion thicker than the moving part.
With the above arrangement, the limitation on the size of the material substrate, or the wafer, used for manufacturing the micro mirror unit is reduced. The micro mirror unit according to the first aspect of the present invention includes a frame which has a portion thicker than the moving part. Therefore, even if the mass of the entire moving part, the amount of movement of the moving part, the size of the comb-like electrodes necessary for achieving the amount of movement and so on require the moving part to have a first thickness as thin as 100 through 200 xcexcm for example, it is still possible to use a wafer having a second thickness thicker than the first thickness, in the manufacture of the micro mirror unit. When using such a wafer, the second thickness is maintained in a predetermined or larger area of the frame throughout steps for forming necessary members of the element, whereby the strength of the wafer can be maintained. As a result, it becomes possible to appropriately prevent the wafer from being destroyed, in the manufacturing process of the micro mirror unit.
As described, the micro mirror unit according to the first aspect of the present invention includes a frame which has a portion thicker than the moving part. This means that the frame extends beyond the moving portion at least on one side thickness-wise of the element. Therefore, if the frame extends sufficiently on the side away from the mirror surface of the moving part, it becomes possible to mount the micro mirror unit directly onto a wiring substrate via the frame. This is because the frame extending sufficiently provides appropriate space between the moving part and the wiring substrate, and as a result, the movement of the moving part is not hindered by the wiring substrate. On the other hand, if the frame extends sufficiently on the same side as is the mirror surface of the moving part, it becomes possible to bond a transparent cover such as a glass plate directly onto the micro mirror unit to protect the mirror surface. This is because the frame extending sufficiently provides appropriate space between the moving part and the transparent cover, and as a result, the movement of the moving part is not hindered by the transparent cover.
As described, according to the micro mirror unit offered by the first aspect of the present invention, it is possible to reduce the limitation on the size of the flat surface of the wafer used for the manufacture. Further, it becomes possible to appropriately bond adjacent members such as a wiring substrate and a transparent cover without using spacers prepared separately.
According to a second aspect of the present invention, there is provided another micro mirror unit comprising a moving part, a frame and a torsion bar connecting the moving part to the frame. The moving part, the frame and the torsion bar are formed integral from a material substrate having a layered structure including an intermediate layer and silicone layers sandwiching the intermediate layer.
The moving part includes: a first intermediate portion originating from the intermediate layer; a first structural member held in contact with the first intermediate portion and provided with a mirror portion; and a second structural member held in contact with the first intermediate portion on a side opposite to the first structural member.
The frame includes: a second intermediate portion originating from the intermediate layer; a third structural member held in contact with the second intermediate portion on a same side as the first structural member; and a fourth structural member held in contact with the second intermediate portion on a same side as the second structural member, and
The fourth structural member extends beyond the second structural member in a layering direction of the layered structure.
An micro mirror unit having such an arrangement can also reduce the limitation on the size of the flat surface of the wafer used for the manufacture as described for the first aspect. Further, again as described for the first aspect, it is possible to appropriately bond adjacent members such as a wiring substrate without using separate spacers. A preferred embodiment of the micro mirror unit according to the second aspect further comprises a wiring substrate bonded to the fourth structural member.
Preferably, the micro mirror unit may further comprise a wiring substrate bonded to the fourth structural member. Also, the third structural member may extend beyond the first structural member in the layering direction.
According to a third aspect of the present invention, there is provided a micro mirror unit comprising a moving part, a frame and a torsion bar connecting the moving part to the frame. The moving part, the frame and the torsion bar are formed integral from a common material substrate having a layered structure including an intermediate layer and silicone layers sandwiching the intermediate layer.
The moving part includes: a first intermediate portion originating from the intermediate layer; a first structural member held in contact with the first intermediate portion and provided with a mirror portion; and a second structural member held in contact with the first intermediate portion on a side opposite to the first structural member.
The frame includes: a second intermediate portion originating from the intermediate layer; a third structural member held in contact with the second intermediate portion on a same side as the first structural member; and a fourth structural member held in contact with the second intermediate portion on a same side as the second structural member.
The third structural member extends beyond the first structural member in a layering direction of the layered structure.
Preferably, the micro mirror unit may further comprise a transparent cover bonded to the third structural member.
Preferably, in the respective micro mirror units described above, the moving part may include a first comb-like electrode, and the frame may include a second comb-like electrode for operation of the moving part by static electric force generated between the first and the second comb-like electrodes.
Preferably, the first comb-like electrode may be formed in the first structural member, and the second comb-like electrode may be formed in the fourth structural member at a portion contacting the second intermediate portion.
Preferably, in the respective micro mirror units described above, the moving part may include: a relay frame connected to the frame via the torsion bar; a mirror-formed portion spaced from the relay frame; and a relay bar connecting the relay frame to the mirror-formed portion, the relay bar extending in a direction across a direction in which the torsion bar extends.
In the above case, the mirror-formed portion may include a third comb-like electrode, and the relay frame may include a fourth comb-like electrode for operation of the mirror-formed portion by static electric force generated between the third and the fourth comb-like electrodes. The third comb-like electrode may be formed in the first structural member, while the fourth comb-like electrode may be formed in the second structural member.
According to a fourth aspect of the present invention, there is provided a method for making a micro mirror unit provided with a moving part, a frame and a torsion bar. The method includes the steps of:
performing first etching to a material substrate in a thickness direction of the substrate by using a first masking pattern and a second masking pattern, the first masking pattern being arranged to mask a region of the substrate that is to become at least a part of the frame, the second masking pattern being provided with a portion for masking a region of the substrate that is to become the moving part;
removing the second masking pattern; and
performing second etching to the material substrate by using the first masking pattern.
Preferably, the first etching may be performed midway in the thickness direction of the substrate, the second etching being performed to penetrate the material substrate so that at least the moving part is formed.
Preferably, the first etching may be performed until the material substrate is penetrated, the second etching being performed midway in the thickness direction of the substrate so that at least the moving part is formed.
According to a fifth aspect of the present invention, there is provided a method for making a micro mirror unit from a material substrate that includes a first silicon layer, a second silicon layer and an intermediate layer sandwiched between these silicon layers. The micro mirror unit to be produced includes a moving part, a frame and a torsion bar. The method includes the steps of:
performing first etching to the first silicon layer of the material substrate by using a first masking pattern and a second masking pattern, the first masking pattern being arranged to mask a region of the first silicon layer that is to become at least a part of the frame, the second masking pattern including a portion for masking a region of the first silicon layer that is to become the moving part;
removing the second masking pattern; and
performing second etching to the first silicon layer by using the first masking pattern.
Preferably, the first etching may be performed midway in a thickness direction of the first silicon layer, the second etching being performed until the intermediate layer is reached.
Preferably, the first etching may be performed until the intermediate layer is reached, and the second etching may be performed midway in a thickness direction of the first silicon layer.
Preferably, the second masking pattern may further include a portion for masking a region of the first silicon layer that is to become a comb-like electrode in the frame.
According to a sixth aspect of the present invention, there is provided a method for making a micro mirror unit by using a first material substrate including a first silicon layer, a second silicon layer and an intermediate layer sandwiched between these silicon layers, the micro mirror unit including a moving part, a frame and a torsion bar. The method includes the steps of:
forming a first masking pattern including a portion for masking a region of the first silicon layer that is to become the moving part;
making a second material substrate incorporating the first masking pattern, by bonding a third silicon layer to a surface of the first silicon layer upon which the first masking pattern is formed;
performing first etching to the third silicon layer by using a second masking pattern including a portion for masking at least a part of the frame, the first etching being continued until the first silicon layer is reached; and
performing second etching to the first silicon layer exposed by the first etching, the second etching being performed by using the first masking pattern until the intermediate layer is reached.
Preferably, the first masking pattern may further include a portion for masking a region to become a comb-like electrode formed in the frame.
According to a seventh aspect of the present invention, there is provided a method for making a micro mirror unit that includes a moving part, a frame provided with a comb-like electrode and a torsion bar connecting the moving part to the frame. The method includes the steps of:
performing first etching to a first silicon layer prepared as a first material substrate, the first etching being performed by using a first masking pattern including a portion to mask a region of the first material substrate that is to become the comb-like electrode, the first etching being continued until the etching reaches a depth corresponding to a thickness of the comb-like electrode;
making a second material substrate that includes the first material substrate, an intermediate layer held in contact with the first material substrate, and a second silicon layer held in contact with the intermediate layer;
performing second etching to the first silicon layer by using a second masking pattern and a third masking pattern, the second masking pattern including a portion to mask a region to become at least a part of the frame, the third masking pattern including a portion to mask a region to become the moving part and the comb-like electrode, the second etching being continued until the etching reaches a midway portion of the first silicon layer;
removing the third masking pattern; and
performing third etching to the first silicon layer by using the second masking pattern until the comb-like electrode is reached.
According to an eighth aspect of the present invention, there is provided a method for making a micro mirror unit by using a first material substrate including a first silicon layer, a second silicon layer and an intermediate layer sandwiched between these silicon layers, the first silicon layer incorporating a torsion bar held in contact with the intermediate layer, the micro mirror unit including a moving part, a frame and the torsion bar. The method includes the steps of:
forming a first masking pattern on the first silicon layer, the first masking pattern including a portion to mask a region to become the moving part;
making a second material substrate incorporating the first masking pattern, by bonding a third silicon layer to a surface of the first silicon layer upon which the first masking pattern is formed;
performing first etching to the third silicon layer by using a second masking pattern including a portion to mask a region to become at least a part of the frame, the etching being continued until the first masking pattern is exposed; and
performing second etching to the first silicon layer by using the first masking pattern until the intermediate layer is reached.
The methods according to the fourth through the eighth aspects of the present invention enable manufacture of the micro mirror units according to the first through the third aspects of the present invention. Therefore, according to the methods offered by the fourth through the eighth aspects, it is possible to reduce the limitation on the size of the flat surface of the wafer used for the manufacture. Further, it is possible to appropriately bond adjacent members to the manufactured element without using separate spacers.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.